فَحِفْظُ الْكِتَابِ حِفْظٌ لِلْمَعْرِفَةِ.
Safeguarding a book is safeguarding knowledge.
A concise expression connects a practical instruction with a broader intellectual value. It supplies motivation and justification without listing each consequence of preserving a book. Its force lies in condensed meaning, not in an independently established omitted clause.
Ījāz al-qaṣr need not posit a missing syntactic component. Ījāz al-ḥadhf requires a plausible omitted element and a clue allowing its recovery. These should not be confused simply because both are brief.